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JPEGWeb Design in a Nutshell, eMatter EditionChapter 15JPEGCHAPTER 15
JPEG Format
JPEG (which stands for Joint Photographic Experts Group, the standards body that
created it) is a compression algorithm used by files in the JFIF format, commonly
referred to as “JPEG files.”
Like any graphics file format to find widespread use on the Web, JPEGs are plat-
form-independent. In addition, JPEGs are fully supported for use as inline images
in versions 2.0 and higher of Netscape Navigator and Microsoft Internet Explorer,
as well as in most other current browsers (see the “JPEGs in AOL Browsers”
section in this chapter).
24-bit Color
JPEG images contain 24-bit RGB color information, which means they are
comprised from the true color space of millions of colors. JPEG files can also carry
grayscale images. This results in higher image quality and more rich and subtle
color variations. Unlike GIF files, JPEGs do not use palettes for referencing color
information.
Bear in mind, however, that when JPEGs are displayed on a system that only
supports 8-bit color, the browser will reduce the colors in the image to the Web
Palette, and some dithering will occur. In general, however, dithering is more
acceptable in photographic image areas than in areas of flat color. For an explana-
tion of the Web Palette, see Chapter 3,Web Design Principles for Print Designers,
and Chapter 17,Designing Graphics with the Web Palette.
JPEG Compression
JPEG uses what is known as a “lossy” compression scheme, meaning that some
color information is actually thrown out in the compression process. Fortunately,
for photographic images at most compression levels, this loss is not discernible to